The present invention is in the field of optoelectronics, including semiconductor laser diodes and semiconductor laser diode based optical amplifiers. In particular, the present invention relates to a novel apparatus and a method for signal modulation and control of signal coupling in semiconductor laser diodes and semiconductor laser diode optical amplifiers.
Fiber optics and semiconductor lasers are now essential telecommunications technologies. Given the drastic increase in demand for high speed information transfer, the increased use of high bandwidth optical communications is a natural solution to increasing bandwidth demands. Fiber optic systems are capable of transferring high symbol rate signals over long distances with low attenuation using dielectric waveguides in the form of optical fibers. Optical fibers are cylindrical dielectric waveguides and offer a better bitrate times distance product than copper wires for a given attenuation or signal to noise ratio. However, in order to derive maximum usefulness from optical sources and ensure the highest data transfer capabilities, high bandwidth, high extinction ratio modulation techniques and device structures are critical. Such techniques and structures must be directed toward maximizing the signal to noise ratio at the receiver or receivers.
Methods for modulating the beams of laser diodes, optical amplifiers, and other optical devices are instrumental for facilitating the high-speed transfer of information across optical conduits. Improved modulation methods further are useful in minimizing factors such as signal loss due to poor signal coupling from one device to another. Prior art modulation of semiconductor laser diodes typically involves direct modulation of the injection luminescence current or external modulation of the optical beam usually through a Mach-Zehnder (MZ) interferometer or device of like kind.
Integrated MZ interferometers or related devices use a single mode input waveguide which is split into two branches of equal or nearly equal length and then recombined into a single mode waveguide. When a single bias voltage or multiple bias voltages are applied to one or both of the branches, a phase difference occurs between the two optical signals. A controllable amount of constructive or destructive interference occurs when these two optical signals are combined in such a manner. By controlling the amount of bias voltage applied to the branches, and thus the amount of constructive or destructive interference between signals, the amplitude of the recombined output signal can be modulated.
MZ based modulators must be constructed from an electro-optic material in which the refractive index is a function of applied voltage such as lithium niobate. Because such materials tend to be relatively costly, external MZ or related modulators are extremely expensive compared to direct modulation.
Direct modulation of a semiconductor laser by varying the magnitude of the applied injection luminescence current imposes an amplitude variation in the laser element. Controlled amplitude variations constitute the signal. While direct modulation is effective at signal constitution, it does not affect nor control the direction of the optical beam. Direct modulation may also cause transient, undesired wavelength shifts commonly referred to as xe2x80x9cchirpxe2x80x9d and capable of degrading pulse shape when the optical signal travels in a dispersive media such as optical fiber.
Direct modulation also restricts dynamic range and reduces the extinction ratio of the signal since the modulation current is varied about a bias point nearly midway between the laser threshold current and the maximum current for safe operation. Dynamic range refers to the ratio of maximum output signal power to the minimum output signal power subject to a signal fidelity criteria such as 1 dB compression and usually applied to analog transmission. Extinction ratio refers to the ratio of peak output with an applied input signal set at a maximum level to the minimum output power with no input signal applied. Thus the limited dynamic range and extinction ratio of prior art modulation techniques limit performance when used to conduct optical telecommunications.
For example, if the sum of the bias and modulation currents are below laser threshold, as they typically are during low signal level constitution, the optical signal is extinguished. During high signal level generation, the signal may be xe2x80x9cclippedxe2x80x9d. Clipping is problematic for analog optical signals requiring signal fidelity. Conversely, if the bias point is selected closer to the maximum safe current to avoid low-level clipping, device reliability may be impaired and xe2x80x9cidle channelxe2x80x9d noise may be increased. Idle channel noise may be particularly problematic in multichannel optical communications.
If many such high-biased lasers are interconnected over an optical network for example, the aggregate optical noise degrades the performance of signal processing devices such as signal demultiplexers and detectors supporting the operation of the network. High bias levels however do increase the intrinsic speed of the semiconductor lasers since the relaxation frequency increases with optical power. Moreover, direct modulation at high bit rates or high frequencies requires fast driver circuits capable of handling relatively large currents since the entire device is xe2x80x9cpumpedxe2x80x9d with current. Providing both high speed and high bias levels within a driver circuit complicates the design of the driver. High speed operation using direct modulation may also be limited by device capacitances.
Capacitances inherently present as an unavoidable consequence of the physical design and construction of a semiconductor device may affect the operation of the device especially at high speeds. High speed operation therefore requires optimization of junction design to reduce parasitic device capacitances. Such optimization usually requires, among other measures, minimizing the length of the device.
Prior art methods for optimizing transmission include beam control for focusing and steering. Such methods are disclosed, for example in U.S. Pat. No. 5,524,013 issued to Nakatsuka, et al on Jun. 4, 1996. Nakatsuka, et al discloses a Beam Scannable Laser Diode wherein the position and emitting direction of a laser beam can be varied by controlling injection currents. By modulating injection currents, the electron distribution and refractive index profile for the optical media may be modified. Nakatsuka, et al relies on changing the refractive index profile to bend an incumbent beam in a manner similar to a gradient index lens by lateral diffraction. One disadvantage of Nakatsuka, et al is that the device is gain guided in the lateral dimension giving rise to astigmatism leading to poor beam quality. Moreover, gain guiding diminishes the beam steering benefits when such a device is operated at low power. Another disadvantage of Nakatsuka, et al is the need for an integral lens. Not only does such a lens require additional dry etching steps to construct, but the use of such a lens leads to optical aberrations including undesirable xe2x80x9ccomaxe2x80x9d.
U.S. Pat. No. 5,319,659 issued to Hohimer on Jun. 7, 1994 discloses a Semiconductor Diode Laser Having an Intra cavity Spatial Phase Controller for Beam Control and Switching. In such prior art laser devices, steering and switching of a single mode signal are accomplished by means of integrating Intra cavity controllers disposed within an electrical contact metallization layer during laser fabrication. Hohimer however does not accommodate multiple lateral waveguide modes.
To best appreciate method and apparatus in the claimed invention disclosed hereinafter, an understanding of the fundamental characteristics of the dielectric waveguide are essential. For a complete understanding of the underlying theory of dielectric optical waveguides, xe2x80x9cTheory of Dielectric Optical Waveguidesxe2x80x9d, Dietrich Marcuse, Academic Press 1974, incorporated herein by reference, may be referred to. In a dielectric slab waveguide, electromagnetic energy propagates within a region of high permittivity surrounded by dielectric material of a lower permittivity. Because of the simplicity of construction and geometry, slab waveguides can be easily characterized with mathematical expressions and are regularly used in integrated optical devices. Such simplicity therefore makes predicting the behavior of electromagnetic energy within the slab waveguide relatively easy using known mathematical relationships.
A dielectric waveguide is a structure in which electromagnetic energy is confined to and propagates in a region of higher permittivity surrounded by dielectric material of lower permittivity. The behavior of the fields in dielectric waveguides is described in part using the following well known equations:                               Gauss          '                ⁢        s        ⁢                  xe2x80x83                ⁢        Law                            xe2x80x83                                          ∇                      ·                                          D                ·                            →                                      =        ρ                            (        1        )                                          Faraday          '                ⁢        s        ⁢                  xe2x80x83                ⁢        Law                            xe2x80x83                                          ∇                      xc3x97                                          E                ·                            →                                      =                              -                          ∂                              B                →                                                          ∂            t                                              (        2        )                                No        ⁢                  xe2x80x83                ⁢        Magnetic        ⁢                  xe2x80x83                ⁢        Monopoles                            xe2x80x83                                          ∇                      ·                                          B                ·                            →                                      =        0                            (        3        )                                          Ampere          '                ⁢        s        ⁢                  xe2x80x83                ⁢        Law                            xe2x80x83                                          ∇                      xc3x97                                          H                ·                            →                                      =                                            J              ·                        →                    +                                    ∂                              D                →                                                    ∂              t                                                          (        4        )            
where {right arrow over (D)} is the electric flux density vector, {right arrow over (B)} is the magnetic flux density vector, {right arrow over (E)} is the electric field vector, {right arrow over (H)} is the magnetic field vector, xcfx81 is the free charge source density, and {right arrow over (J)} is the current source density vector. The flux density vectors are related to the field vectors by the constitution equations:                                           D            ·                    →                =                                            ϵ              0                        ⁢                                          E                ·                            →                                +                                    P              ·                        →                                              (        5        )                                                      B            →                    ·                =                              μ            0                    ⁡                      (                                                            H                  →                                ·                            +                                                M                  →                                ·                                      )                                              (        6        )            
where xcex50=8.854xc2x710xe2x88x9212 Farads/meter and xcexc0=4xcfx80xc2x710xe2x88x927 Henrys/meter are the permittivity and permeability of free space, respectively.
For the special case of non magnetic media, the magnetization field {right arrow over (M)}={right arrow over (0)} so {right arrow over (B)}=xcexc0{right arrow over (H)}. In linear isotropic dielectric media, the relation between the electric field and electric polarization is given by:                                           P            →                    ·                =                              ϵ            0                    ⁢                      χ            e                    ⁢                                    E              →                        ·                                              (        7        )                                                      D            →                    ·                =                              ϵ            ⁡                          (                              x                ,                y                ,                z                ,                            )                                ⁢                                    E              →                        ·                                              (        8        )            xe2x80x83xcex5(x,y,z)=(1+"khgr"e(x,y,z))xcex50xe2x80x83xe2x80x83(9)
where "khgr"e is called the electric susceptibility and (1+"khgr"e(x, y, z)) is the relative permittivity. In source free media, xcfx81=0 and J=0. Accordingly, the curl of Faraday""s law (Equation 2) and of Ampere""s law (Equation 4) may be combined with the vector identity xe2x96xa12{right arrow over (V)}=xe2x96xa1(xe2x96xa1xc2x7{right arrow over (V)})xe2x88x92xe2x96xa1xc3x97xe2x96xa1xc3x97{right arrow over (V)} to yield the vector wave equation:                                           (                                                            ∂                  2                                                  ∂                                      x                    2                                                              +                                                ∂                  2                                                  ∂                                      y                    2                                                              +                                                ∂                  2                                                  ∂                                      z                    2                                                                        )                    ⁢                                    V              ·                        →                          =                              μ            0                    ⁢          ϵ          ⁢                                                    ∂                2                            ⁢                              V                →                                                    ∂                              t                2                                                                        (        10        )            
which holds for each of the components of {right arrow over (V)}, where {right arrow over (V)} is either {right arrow over (E)} or {right arrow over (H)}. Specializing further to the case of time-harmonic waves, the temporal dependence of all three vector quantities Vq is given by Vq (x, y, z, t)="psgr"q (x, y, z) ejxcfx89t and Equation (10) reduces to the Helmholtz equation for phasor fields:                                           (                                                            ∂                  2                                                  ∂                                      x                    2                                                              +                                                ∂                  2                                                  ∂                                      y                    2                                                              +                                                ∂                  2                                                  ∂                                      z                    2                                                                        )                    ⁢                      ψ            q                          =                              -                                          k                2                            ⁡                              (                                  x                  ,                  y                  ,                  z                                )                                              ⁢                      ψ            q                                              (        11        )            
where k2 (x, y, z)=xcfx892xcexc0xcex5 (x, y, z).
When time-harmonic waves propagate in a uniform waveguide, the overall dependence on time and longitudinal distance, z, is given by exp{j(xcfx89txe2x88x92xcex2(xcfx89)z)}, where xcex2(xcfx89) is a modal propagation constant yet to be determined and xcfx89=2xcfx80c/xcex where xcex is a particular wavelength of light in free space. Since ∂2"psgr"z/∂z2=xe2x88x92xcex22(xcfx89)"psgr"z, the scalar Helmholtz equation for the z component of the electric field can be expressed as a two dimensional time-independent Schrxc3x6dinger equation:
xe2x96xa1xe2x8axa52"psgr"z+k2(x, y)"psgr"z=xcex22(xcfx89)"psgr"zxe2x80x83xe2x80x83(12)
where xe2x96xa1xe2x8axa5={right arrow over (x)}(∂/∂x)+{right arrow over (y)}(∂/∂y) is the perpendicular gradient operator and xe2x96xa1xe2x8axa52=∂2/∂x2+∂2/∂y2 is the two dimensional Laplacian operator. Equation (12) must be solved by utilizing the boundary conditions between materials of different permittivity.
Since Maxwell""s equations allow us to express the transverse field components {right arrow over (E)}xe2x8axa5xe2x89xa1({right arrow over (x)}Ex+{right arrow over (y)}Ey) and {right arrow over (H)}xe2x8axa5xe2x89xa1({right arrow over (x)}Hx+{right arrow over (y)}Hy) in terms of the longitudinal components Ez and Hz, it is only necessary to solve Equation (11) once. If Ez=0 then the non zero electric field components are transverse to the direction of propagation; such modes are called xe2x80x9ctransverse electricxe2x80x9d or TE modes. Similarly, if Hz=0 then the magnetic field and the modes xe2x80x9ctransverse magneticxe2x80x9d or TM. Modes which have non zero longitudinal components of both the electric and magnetic fields and which have the same propagation constant, xcex2, are called hybrid modes.
Using known mathematical behavior of electromagnetic waves and, in particular, the behavior of such waves in a dielectric slab waveguide, signals may be modulated in a number of ways. Such modulation may be determined by the method which optimizes, for example, the coupling efficiency for a particular media.
Optimization of optical coupling and transmission may also be affected by beam alignment between coupled devices. Notwithstanding advances in the art, the alignment between optical fibers, semiconductor lasers, and optical amplifiers is extremely difficult due to the small alignment tolerances demanded. Active alignment is typically performed using the optical signal from the device and a position control loop. Using feedback information related to signal intensity, a position control loop may affect a change in the beam alignment to optimize the optical power coupled to a fiber. Active alignment techniques require expensive alignment equipment and highly trained personnel and thereby reduce productivity in module manufacture.
It is an object therefore of the present invention to provide a method and apparatus for overcoming the disadvantages in the prior art. Specifically, it is an object of the present invention to provide a method and apparatus for beam steering that consumed little additional power, that provided high modulation bandwidth, and that provided high dynamic range and large extinction ratios with an integrated beam structure.
It is another object of the present invention to provide a method and apparatus for aligning the external optical components and one or more beams of a semiconductor laser or optical amplifier to provide optimal coupling without resorting to positioner-based active alignment techniques.
It is still another object of the present invention to provide a method for beam steering without resorting to multiple radiating elements.
It is still another object of the present invention to provide a method for beam steering using index guiding and carrier anti-guiding effect thus avoiding beam astigmatism caused by internally refracting the beam using gain guiding.
It is still another object of the invention to provide a method and apparatus for beam steering that did not rely on thermal effects since such effects compromise modulation speed.
The intermodal phase difference controller of the present invention uses a novel method of beam steering to modulate the optical signal of semiconductor lasers or optical amplifiers. Beam steering changes the direction of the optical signal and thus controls the coupling of the semiconductor laser or optical amplifier to the transmission medium and/or other optical elements such as the receiver. The intermodal phase difference controller of the present invention utilizes effective signal coupling and modulation from a single mode laser or amplifier signal propagating in a single mode waveguide to multiple modes in a multi-mode waveguide integral to the laser or amplifier. The intermodal phase difference controller is capable of achieving high signal extinction ratios because the beam can be steered away from other optical elements. Beam steering is capable of achieving high speed, low chirp modulation since the semiconductor laser or optical amplifier section is biased independently of the smaller beam steering section.
The beam steering method of this invention offers the advantages of external modulation, namely, reduced amplitude to phase (AM-to-PM) conversion thereby producing nearly chirp-free operation. The beam steering method of the present invention also offers a reduction in the amount of junction and parasitic capacitances through reduction in the area subject to the modulating current. By lowering capacitance, high speed modulation is accordingly enabled.
The beam steering method of the present invention maybe practiced on a device comprising a single vertical and lateral mode optical waveguide, a mode converter, a multi-lateral mode waveguide, and controlling electrodes. All waveguide modes are fundamental in the direction normal to the semiconductor layer structure and semiconductor epitaxial surface and may be referred to as vertical mode. Transmission in the vertical mode is often referred to as the xe2x80x9ctransversexe2x80x9d mode in the semiconductor laser diode literature. The waveguide""s lateral mode structure, e.g., single or multi-lateral modality, further may be referenced to the plane of the semiconductor laser layer structure. Transmission mode may also be referred to as perpendicular and parallel to the junction plane, respectively.
The mode converter of the present invention efficiently couples the output of a single mode waveguide to two or more lateral modes of a multi-lateral mode waveguide. In one embodiment, a mode converter couples the single mode, output nearly equally to a multi-lateral mode waveguide supporting only two guided modes. The two guided modes travel in a multi-lateral mode with slightly different velocities due to the modal dependence of waveguide dispersion also referred to as modal dispersion. The two guided modes therefore arrive at the device facet with a particular intermodal phase difference based on the initial mode phasing, the length and the modal dispersion properties of the multi-lateral mode waveguide, and the angle of the facet with respect to the multi-mode waveguide.
Beam steering is effected by correctly injecting the multi-lateral mode waveguide from the mode converter and by changing the intermodal dispersion. The modal dispersion can be changed using the carrier antiguiding effect to inject current in the multi-lateral mode index guided waveguide. Carrier antiguiding effect is the reduction in the refractive index in the semiconductor by increasing carrier density. The change in the refractive index affects the individual modal phase velocities, confinement factors, and the difference between individual phase velocities. The intermodal phasing at the device facet can be controlled with both or either of these effects. Angling the facet with respect to the waveguide establishes an initial beam pointing direction, changes the modal reflectance, and increases intermodal coupling in the reflected signal. The intermodal phase difference changes the direction that the beam propagates relative to the device facet, effecting the beam steering method of the present invention.